192
9 Molecular Flexibility and Material Properties
the presence of surrounding rigid cores, resulting in a reduction in the chain entropy.
At least, the tendency to increased entropy would disfavor the mixed layer.
As described in the preceding section, the SmA phase in nCB continuously swells
with alkanes [71]. The increment in the layer spacing is consistent with that calculated
while assuming that the neat liquid alkane preserves its density in the swelled state.
Thus, it is highly plausible that the molten chains, together with “real” solvent,
liquid alkane, form separate layers from those of aromatic cores. This structural
model implies that the molten chains and cores separately form layers even in the
neat system, as shown in Fig. 9.10b. The similarity of the quasi binary systems of
thermotropic liquid crystals with real lyotropic systems extends over not only the
phase behavior but also the formed structure. Mostly the same local aggregation
structure (as expressed in terms of the ratio between the molecular length and the
average repeat distance between molecules) in the N phase of 5CB and 7CB [83,
84] seems consistent with the “lyotropic” formation of the SmA phase in neat nCB
and nCB-alkane systems. Note that the view of “lyotropic” nature for the formation
of smectic phases is historically not entirely new [85–88]. Indeed, Guillon et al.[87,
88] reported some experimental works showing that the core and flexible chains
segregated though weakly.
The decisive evidence of the nano-segregated structure of smectic phases comes
from the investigation of the SmE phase [89, 90]. Although the orientation and
conformation of molecules are highly disordered, the SmE phase has the threedimensional periodicity, which means that well-defined lattice constants characterize
its averaged structure [91, 92]. If the SmE phase of a mesogen with the core-chain
structure swells with alkane, the swelling itself serves as supporting evidence. The
phase diagram determined for the binary system of 9TCB (the inset in Fig. 9.7) and
n-nonane indicates the continuous swelling of the SmE phase structure [89]. Further,
its three-dimensional periodicity offers stronger evidence. The lattice constant along
the stacking direction (along the molecular long axis) increases continuously upon
swelling, while those within the layer scarcely depend on the extent of swelling.
No effect on the structural order within the layer indicates the invasion of alkane
molecules between cores is negligible, and all alkane molecules reside within the
chain layers. The three-dimensional periodicity of the SmE phase also enables an
attempt to analyze its aggregation structure in neat nTCBs [90], even though the
full (three-dimensional) analysis of SmE structures through the X-ray diffraction is
impossible because of severe structural disorder. The so-called Fourier syntheses
3
of series of diffraction peaks assignable to the stacked layers (up to fifth order) of
aligned samples indicate the stepped electron density along the stacking axis (layer
normal). The length of the high electron density region remains constant and equal to
the length of the core part for a broad range of the alkyl chain length (2 ≤ n ≤ 10).
These results indicate that the structure of the analyzed SmE phase of the nTCB
series is described as not the stack of uniform molecular layers but as the alternate
stack of the core and chain layers (Fig. 9.10b).
3 Remember that the square root of the intensity of diffraction is a Fourier transform of the density
of scatterer as explained in Sect. 4.2.
9 Molecular Flexibility and Material Properties
the presence of surrounding rigid cores, resulting in a reduction in the chain entropy.
At least, the tendency to increased entropy would disfavor the mixed layer.
As described in the preceding section, the SmA phase in nCB continuously swells
with alkanes [71]. The increment in the layer spacing is consistent with that calculated
while assuming that the neat liquid alkane preserves its density in the swelled state.
Thus, it is highly plausible that the molten chains, together with “real” solvent,
liquid alkane, form separate layers from those of aromatic cores. This structural
model implies that the molten chains and cores separately form layers even in the
neat system, as shown in Fig. 9.10b. The similarity of the quasi binary systems of
thermotropic liquid crystals with real lyotropic systems extends over not only the
phase behavior but also the formed structure. Mostly the same local aggregation
structure (as expressed in terms of the ratio between the molecular length and the
average repeat distance between molecules) in the N phase of 5CB and 7CB [83,
84] seems consistent with the “lyotropic” formation of the SmA phase in neat nCB
and nCB-alkane systems. Note that the view of “lyotropic” nature for the formation
of smectic phases is historically not entirely new [85–88]. Indeed, Guillon et al.[87,
88] reported some experimental works showing that the core and flexible chains
segregated though weakly.
The decisive evidence of the nano-segregated structure of smectic phases comes
from the investigation of the SmE phase [89, 90]. Although the orientation and
conformation of molecules are highly disordered, the SmE phase has the threedimensional periodicity, which means that well-defined lattice constants characterize
its averaged structure [91, 92]. If the SmE phase of a mesogen with the core-chain
structure swells with alkane, the swelling itself serves as supporting evidence. The
phase diagram determined for the binary system of 9TCB (the inset in Fig. 9.7) and
n-nonane indicates the continuous swelling of the SmE phase structure [89]. Further,
its three-dimensional periodicity offers stronger evidence. The lattice constant along
the stacking direction (along the molecular long axis) increases continuously upon
swelling, while those within the layer scarcely depend on the extent of swelling.
No effect on the structural order within the layer indicates the invasion of alkane
molecules between cores is negligible, and all alkane molecules reside within the
chain layers. The three-dimensional periodicity of the SmE phase also enables an
attempt to analyze its aggregation structure in neat nTCBs [90], even though the
full (three-dimensional) analysis of SmE structures through the X-ray diffraction is
impossible because of severe structural disorder. The so-called Fourier syntheses
3
of series of diffraction peaks assignable to the stacked layers (up to fifth order) of
aligned samples indicate the stepped electron density along the stacking axis (layer
normal). The length of the high electron density region remains constant and equal to
the length of the core part for a broad range of the alkyl chain length (2 ≤ n ≤ 10).
These results indicate that the structure of the analyzed SmE phase of the nTCB
series is described as not the stack of uniform molecular layers but as the alternate
stack of the core and chain layers (Fig. 9.10b).
3 Remember that the square root of the intensity of diffraction is a Fourier transform of the density
of scatterer as explained in Sect. 4.2.